Various optical devices, employed to process laser beams, may introduce optical discontinuities in the path of the laser beam. Such optical discontinuities tend to produce diffraction effects, so that diffraction peaks are introduced onto the laser beam. Such diffraction peaks are undesirable and can rise to intensity levels which are sufficiently great to cause damage to the laser system.
Although various optical devices may introduce such optical discontinuities, one specific example will be discussed in some detail. Such example may take the form of a harmonic conversion device having a multi-element crystal array for a large aperture laser system. Such harmonic conversion devices are known and used for the purpose of converting a fundamental frequency input laser beam into a second or third harmonic output laser beam, for example. In such devices, the harmonic conversion may be produced by known or suitable crystal materials, such as KDP (potassium dihydrogen phosphate). KDP crystals are grown to fairly large sizes. However, for large aperture laser systems it is generally necessary to employ a multi-element crystal array in a harmonic conversion device. For example, in a large laser system having a 74 cm output aperture, it has been necessary to construct arrays comprising from 9 to 25 square KDP crystal elements. The interstices between the elements introduce optical discontinuities which produce diffraction effects in in the laser beam. A supporting grid often provided between the inerstices between the crystal elements, referred to as an "egg crate", also introduces optical discontinuities which tend to produce diffraction effects, including diffraction peaks.